Lol love this!!
Tuna live in the ocean, so they are not freshwater fish, but unfortunately most water is contaminated with mercury, oceans, rivers and lakes. This link gives a good run down of mercury amounts in various fish: http://www.americanheart.com/presenter.jhtml?identifier=3013797 Salmon is actually pretty low on the contamination list, and I would feel ok with eating it. Look for wild caught Alaskan Salmon, as they are the most sustainably fished.